Automatic welding machines are used in manufacturing and automated production assembly lines to continuously perform welding of cooperating parts. For example, automated resistance spit welding of parts in an assembly line set-up is performed generally by conveyance of the parts to be welded into an automatic welding unit which holds the parts in specified relative positions, places electrodes into contact at appropriate positions upon the parts with compressive force, and applies current to the weld electrodes to flow through the parts held in compression. For assembly line manufacture of a succession of identical end products such as, for example, brake shoes, stand alone machines have been devised which receive, convey and manipulate the parts into the correct position for welded assembly, automatically perform the welding process on the parts at specific weld locations, and conveys the completed welded part on to a subsequent step in the manufacturing process. One such automatic brake shoe welding machine is the Par Industries, Inc. H-BSAR-GS-EPR Hydraulic Automatic DC Brakeshoe Welder manufactured by Hess Engineering, Inc. of Niles, Mich. U.S.A. under license from Ottenser Eisenwerke GmbH Hamburg, West Germany.
Of course, in automatic welding of brake shoes, the quality and strength of the welds is critical both to the performance of the brake shoe and the safety of the vehicle in which it is ultimately installed. To acquire data on the quality and physical strength of welds made by an automatic welding machine, a random sampling of welded assemblies is made for destructive testing and measurement. For example, two brakeshoes may be selected from the production line within a single thirty minute time interval and destructively tested by forced separation of the welds between the rim and web components. Weld nuggets formed by the resistance welding process are then inspected and measured for a minimum content of material broken away from the welded components. Such random testing gives only limited data on the strength and quality of all welds made in a production run, and yields no direct or exact data on the physical conditions under which the welds were formed.
Welding current monitors have been devised to measure voltage, current and time values in resistance welding and compare measured values to defined limits as a form of weld process control. Such monitors do not, however, monitor other physical conditions attendant the welding process such as machine pressures which exert compressive forces upon the parts during weld joining, and pressures applied to weld assembled parts in automated finishing or dimensioning operations. Nor do such monitors correlate weld condition data to the exact parts weld assembled thereunder.
Thus a need exists for real time monitoring of a broad range of physical conditions under which parts are joined by welding performed by an automatic welding machine to monitor machine functions together with weld current, and to provide a complete record of values of conditions monitored correlated to each particular part weld assembled by the machine.